If an Address isn't specified, the next logical address in the External Network address space will be allocated.

Adding External Network IP Addresses to Instances

Once an External Network and External Network IP Address have been created, they can be assigned to an Instance for it to participate in that External Network. The Instance does not need to be shut down for this.

External Network IP Addresses can be added:

When an Instance is being created with the Advanced Create popup. These settings are under the Networks section

When an Instance is created, by clicking the ⚙ Actions button next to the Instance in the Instances screen under Compute, and selecting Networking.

To add the External Network IP Address to a new network interface:

Click "Add network..."

Choose the External Network you wish to add the IP address from

Click "Add IP address..." under the interface with the External Network attached